Abstract: Ecological land provides the biggest part of ecological services;minimum ecological land is the basic line of regional ecological security. Based on the concept of ecological security, the paper took the middle reaches of the Heihe River, northwest China, as an example, calculated the minimum ecological land, which can keep social-economic system running safely, based on modified ecological footprint model, and propound an ecological land optimization scheme combined with ecological importance assessment and ecological suitability assessment. The results are as follows:(1) The supply and demand of ecosystem services are out-of-balance. In 2014, the footprint in the Heihe River is 5.5675 per capita, while the ecological supply is 4.8392 per capita, the ecological deficit is 0.7283 per capita. 3010 km² forest or 9086 km² grassland or 431 km² water area are needed to keep the balance of supply and demand of ecosystem services.(2) The most important ecological region in the study area are mainly distributed at the south of the Qilian Mountain Nature Reserve area and the Heihe Wetland Nature Reserve area, with a total area of 2767 km², the main land cover is forest and grass land. The second important ecological region are mainly distributed at the center of Ganzhou, northeast of Linze, with a total area of 964 km², the main land cover is cultivated land. The important ecological region are mainly distributed at Ganzhou, center of Linze and the north of Shandan horse ranch, with a total area of 3058 km², the main land cover is cultivated land and unused land.(3) Based on the result of the recognition of minimum ecological land, ecological importance and ecological suitability assessment, the optimization of ecological land in the region needs to return 369 km² cultivated land to forest, 191 km² cultivated land to grass land and 141 km² unused land to forest, and the research put forward a space optimization scheme. The result has an important guiding significance for ecological construction and ecological system management in the middle reaches of the Heihe River.

Key words: optimization of minimum ecological land, modified ecological footprint model, ecological importance, ecological suitability assessment, middle reaches of Heihe River